1. Field of the Invention
The present invention relates generally to data communications in a wireless communication system, and more particularly, to systems and methods of providing intelligent cell reselection for delay sensitive applications.
2. Description of the Prior Art
In today's wireless communications environment, data applications have become commonplace and widely used. Certain of these data applications, such as real time and streaming applications, are delay sensitive and have very strict requirements with respect to transfer data rate and jitter. If these requirements are not met, the performance of the application may degrade substantially, leading to, for example, interruptions of, or dropouts in, data (e.g., audio or video) streams.
FIG. 1 illustrates jitter in a wireless communications network. A transmitted burst 100 of data includes multiple packets 110 of data. In a streaming application, such as video streaming from the Internet, the transmitted burst 100 includes data packets 110 transmitted in an orderly and time-sensitive fashion. Radio transmission errors inherent in a wireless communications interface between a cell and a mobile station such as multiuser interference, multipath fading, and shadowing, result in a jittered (i.e., delayed by a variable interval or lag time), received burst 120 at the mobile station. Radio transmission errors may be particularly acute near an edge of a cell or in a region of overlap between adjacent cells.
If the radio transmission errors are recoverable, a data buffer in the mobile station temporarily stores and retimes the received data packets 130, resulting in a retimed burst 140 with data packets 150 resembling the transmitted data packets 110, with no loss in data. However, if the radio transmission errors exceed a capacity of the data buffer, the resulting burst 160 includes data packets 170 and a missing data packet. 175. The radio transmission errors cause complications in the routing of data packets to the mobile station, because lost data packets may need to be retransmitted by the network.
FIG. 2A illustrates routing of data packets in General Packet Radio Service (GPRS)/Edge, in accordance with the prior art. Data packets intended for a mobile station 205 from a public data network such as the Internet reach a gateway GPRS support note (GGSN) 201 associated with a home network of the mobile station 205. Location information related to the mobile station 205 is stored in a GPRS register contained in a home location register (HLR) 210. The GGSN 201 determines a serving GPRS support note (SGSN) 215 serving the mobile station 205 (e.g., the SGSN A 215a), encapsulates the data packets, and forwards (i.e., tunnels) the data packets to the SGSN A 215a via a base station controller (BSC) 217 (e.g., the BSC 217a) to a base station 220 (e.g., the base station 220b) currently serving the mobile station 205. The base station 220 currently serving the mobile station 205 is referred to as the “serving cell.” As the mobile station 205 moves, for example from position 1 to position 2 in FIG. 2A, the mobile station 205 may select another base station 220d as the serving cell (i.e., perform cell reselection or handoff between cells), because each base station 220a-d covers a limited geographic area. As will be discussed with respect to FIG. 2C below, cell reselection may require retransmission of data packets, requiring significant network overhead and time delay in data transmission.
FIG. 2B illustrates a procedure for cell reselection for GPRS/Edge, in accordance with the prior art. At an initial step 225, the mobile station 205 measures a received signal strength of a broadcast control channel of the current serving cell (e.g., the base station 220a) and sixteen strongest neighboring cells 220 of FIG. 2A. At step 260, the mobile station 205 determines if the received signal strength of the current serving cell 220a is below a predetermined threshold. If the received signal strength of the current serving cell 220a is greater than that of its neighbor cells, the mobile station 205 continues to receive data packets from the current serving cell 220a. If the received signal strength of a neighboring cell 220 is sufficiently stronger than the current serving cell 220a, the mobile station 205 performs cell reselection. Generally, a new serving cell 220 represents the strongest received signal strength of the neighboring cells 220. The mobile station 205 determines independently of the network if the neighboring cell 220 is more suitable than the current serving cell 220.
FIG. 2C illustrates graphically a procedure for cell reselection in accordance with the prior art. When the mobile station 205 (FIG. 2A) performs cell reselection, the mobile station 205 changes is routing area. The mobile station 205 sends a routing area update request containing cell identity of the current serving cell (e.g., the base station 220a) and an identity of the existing routing area to a new SGSN 215 (e.g., the SGSN B 215b) of FIG. 2A. The SGSN B 215b requests an old SGSN (e.g., the SGSN A 215a of FIG. 2A) to provide the GGSN 201 (FIG. 2A) address and tunneling information of the mobile station 205. The new SGSN B 215b (FIG. 2A) then updates the GGSN 201 of the home network with the SGSN B 215b address and new tunneling information. The new SGSN B 215b also updates the home location register 210 (FIG. 2A). The home location register 201 cancels an information context for the mobile station 205 in the old SGSN A 215a and loads subscriber data to the new SGSN B 215b. The new SGSN B 215b acknowledges the mobile station 205, and requests the old SGSN A 215a to supply undelivered data for transmission to the mobile station 205 by the new SGSN B 215b. 
A problem with the cell reselection procedure of FIG. 2A-2C is that there may be an outage coincident with cell reselection. Because duration of the cell reselection process can take many seconds (typically more than 2 seconds), cell reselection can introduce operationally significant delays in data transmission, leading to lost data packets and degradation of a delay-sensitive data application running on the mobile station 205. Lost data packets may require re-queuing of the data packets in the GGSN 201 or SGSN 215, or may result in dropped data packets (e.g., missing frames of audio/video) for the delay-sensitive data application. For example, in real time video applications, the cell reselection procedure may result in breaks in video feed and slow video updates.
A further problem with the cell reselection procedure of FIGS. 2B and 2C is that the mobile station 205 makes a determination of whether to reselect the serving cell independently of the network (i.e., the GGSN 201, the SGSN 215, and the cells 220). Because the cell reselection is independent of the network, the network may be forced to rapidly respond to the mobile station 205 in a new serving cell 220. This requires significant overhead in the network. The majority of existing wireless data transmission technologies, such as GSM/GPRS, do not adapt the cell reselection algorithm to accommodate the requirements of delay-sensitive applications.
Therefore, a need exists in industry to address the aforementioned deficiencies and inadequacies.